1. Field of the Invention
This invention relates generally to high temperature superconducting materials, components and articles incorporating same, and processes for forming same.
2. Description of the Related Art
High temperature superconducting (HTS) materials have immense potential for use in electric power, electronics, and medical industries. Presently, the HTS material that is manufactured in considerable quantities by industry is based on (Bi,Pb)SrCaCuO (BSCCO) superconductor. This material and the process used to manufacture it have proven to be expensive. Furthermore, the properties of this material degrade quickly in the presence of magnetic fields that are generated in a number of electric power devices.
ReBaCuO (Re=rare earth) superconductor is being developed as a potential alternative to the BSCCO superconductor. When fabricated in the form of a thin film coated on a metal substrate (HTS coated conductor), this material exhibits superior current carrying capability compared to BSCCO.
Several deposition processes are being developed to fabricate HTS Coated conductor. In general, thin film deposition techniques can be classified into two major categories: (1) physical vapor deposition (PVD) and (2) chemical processes (see “The Material Science of Thin Films”, Milton Ohring, Academic Press, 1992; S. L. Swartz, IEEE Transactions on Electrical Insulation, 25(5), 1990, 935; S. B. Krupanidhi, J. Vac. Sci. Technol. A, 10(4), 1992, 1569).
Most of the approaches are based on physical vapor deposition techniques where a source of HTS material is vaporized by means of (1) ablation with a high power laser, (2) evaporation using an electron-beam source, or (3) sputtering using high energy argon ions.
However, these techniques are limited in several ways. First, they are all limited by line of sight i.e., the vapors can coat the substrate only where they can ‘see’ the substrate, which means that the coated area is small. This limits the throughput of coated tape. Second, the composition of the coated film is limited to the composition of the material being vaporized. Third, the source material has to be maintained under vacuum causing refill to be difficult and in turn poses a problem for long-length manufacturing. Fourth, the source material has to be formed into a monolith, which adds cost to the process. Fifth, a high vacuum is needed which increases cost of capital equipment
The chemical processes can further be divided into two subgroups i.e., chemical vapor deposition and wet chemical processes including sol-gel and metalorganic decomposition (MOD).
Generally speaking, wet chemical processes such as sol-gel and MOD for deposition of thin films are popular because of their simplicity. However, wet chemical processes have limitations including rate of conversion of precursor to film, thickness control, need for multiple steps for film formation (deposition, bakeout, & heat treatment as a minimum), need for repeating these multiple steps multiple times to build thick films, carbon residue incorporation in the films, difficulties in epitaxial growth in thick films, and evolution of harmful byproducts such as HF if fluorinated precursors are used.
On the other hand, chemical vapor deposition is a particularly attractive method for forming thin film materials because it is readily scaled up to production runs and because the electronic industry has a wide experience and established equipment base in the use of CVD technology, which can be applied to new CVD processes. In general, the control of key variables such as stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD. Forming the thin films by CVD permits the integration of these materials into existing device production technologies. CVD also permits the formation of layers of materials that are epitaxially related to substrates having close crystal structures.
A wide variety of source materials for chemical processing have been employed to form thin films, layers and coatings on substrates. These source materials include reagents and precursor materials of widely varying types, and in various physical states. Vapor deposition has been used widely as a technique to achieve highly uniform thickness layers of a conformal character on the substrate. In vapor phase deposition, the source material may be of initially solid form that is sublimed or melted and vaporized to provide a desirable vapor phase source reagent. Alternatively, the reagent may be of normally liquid state, which is vaporized, or the reagent may be in the vapor phase in the first instance.
In the liquid delivery approach, the liquid or solid precursor is typically dissolved in a solvent, and the solution is stored, e.g., at ambient temperature and pressure. When the deposition process is to be run, the solution is transported to a high temperature vaporization zone within the CVD system, where the precursor is flash vaporized (along with the solvent), and the gas-phase precursor then is transported to the deposition zone, such as a chemical vapor deposition reactor, containing a substrate on which deposition of the desired component(s) from the vapor-phase precursor composition takes place.
The liquid delivery technique has been found to be useful for deposition of multicomponent oxide thin films such as (Ba,Sr)TiO3, SrBi2 Ta2O9, (SBT), (Pb, La) TiO3, (PLT) and Pb(Zr, Ti)O3 (PZT) for example. In CVD processes developed for these and other compounds, it is desirable to dissolve all the precursors in solution, and vaporize them, following which the vaporized precursor composition containing the respective components is transported to the deposition chamber, as described above.
Liquid delivery systems of varying types are known in the art, and for example are disclosed in U.S. Pat. No. 5,204,314 issued Apr. 20, 1993 to Peter S. Kirlin et al. and U.S. Pat. No. 5,536,323 issued Jul. 16, 1996 to Peter S. Kirlin et al., which describe heated foraminous vaporization structures such as microporous disk elements. In use, liquid source reagent compositions are flowed onto the foraminous vaporization structure for flash vaporization. Vapor thereby is produced for transport to the deposition reactor. The liquid delivery systems of these patents provide high efficiency generation of vapor from which films may be grown on substrates. Liquid delivery systems of such type are usefully employed for generation of multicomponent vapors from corresponding liquid reagent solutions containing one or more precursors as solutes, or alternatively from liquid reagent suspensions containing one or more precursors as suspended species.
The simplicity of such liquid delivery approach has been fortuitous, because each component in this system of metalorganic precursors can be treated identically in the respective solution-forming, vaporization and transport steps of the process.
The present inventor has recognized that CVD using metalorganic precursors (MOCVD) would be a desirable technique for fabrication of films of various materials including HTS. However, MOCVD has yet to be shown to be a viable approach to achieve high current and high current density with HTS Coated conductors because suitable MOCVD apparatuses and processes have not been developed.
In view of the state of the art, it is generally desirable to form HTS conductors and components therefrom having desirable electrical properties, and form such conductors by techniques that are commercially viable.